With switched-mode power supplies a current limiter is almost always incorporated along with the power semiconductor and the coils to provide protection. The consequence of this current limiter is that, when the output power is divided up into a number of arms for protection with a fuse or with an automatic cutout the trip current needed cannot be delivered in the event of a short circuit. Electronic security cutouts with a precise overload characteristic curve and precise tripping current are already available. These circuits are expensive however since with a heavy load a very large amount of leakage power is built up in the switch elements of the security cutout.
It has thus been suggested in the prior art for example to restrict the loads to such types as draw only small start-up overcurrents, and thus remain below the pulse leakage power of the current-limited switches which operate during the start-up phase in linear mode. The mostly pulse-shaped linear operating phases (5 to 200 ms) mean that the leakage power cannot be dissipated from the transistor quickly enough, so that its thermal capacity must exclusively accept the leakage power. A silicon chip with a thickness of around 100 μm used and a surface of a few square millimeters can thus only accept energy to a very limited extent.
It has thus also been proposed in the prior art to also relocate leakage power to an additional resistor, which is controlled via a separate transistor and thus make a significantly greater mass available for energy acceptance. Because of the requirement to implement any given security cutout setting values (adjustable security cutout), in the case of the highest leakage power occurring, only 75% of the energy can be accepted by the resistor. The transistors must thus be able to cover the remaining 25% with their thermal capacity. Such a circuit arrangement is known from WO 02/082611.
A number of power resistors and an electronic switch in series in each case enable an overall resistance between load and supply source to be achieved with the corresponding graduation by selecting the right combination of transistors switched on and switched off, so that the load current does not exceed predeterminable limits.